This invention relates to programmable logic devices that include dedicated multipliers, and more particularly to such programmable logic devices in which the multipliers are used in particular configurations that reduce resource utilization.
It has become more common to provide multiplier circuits on programmable logic devices, rather than requiring users of such devices to construct multipliers from the available programmable logic resources. However, a multiplier circuit consumes a relatively large area, and its inputs can consume significant routing resources.
For example, multipliers are provided to multiply m bits by n bits—e.g., 18×18 bits (frequently m=n). However, a user of the programmable logic device might have need of a p-bit by q-bit multiplier, where p and q are chosen by the user at the time of programming and may be different in every case, and p<m and q<n. This can be accomplished during programming by pre-loading or padding the unused bits with zeroes. However, the inputs to those unused bits have to be driven by a source, and the source has to be routed to the inputs. Therefore, padding the unused bits consumes resources which then are unavailable for other uses, even though the inputs remain constant throughout device operation.
Alternatively, additional registers could be provided and ANDed with the multiplier input registers, and each additional register could be set to either one (this would be the case for the most significant multiplier bits, which will be used) or zero (in the case of the least significant multiplier bits, which will not be used). Whether a particular register was set to zero or one could be controlled by configuration bits. While this consumes fewer resources than routing the zeroes directly to the less significant multiplier inputs, it still requires providing additional registers and configuration bits.
In another example, a multiplier might be used in a configuration in which one of its inputs is a constant coefficient, again consuming routing resources for the constant coefficient. Indeed, one such use is in a finite impulse response (FIR) filter, which requires several multipliers, compounding the use of routing resources. Moreover, in such a filter, the outputs of the various multipliers must be accumulated by a plurality of adders, consuming further routing resources to direct the various products to the adders and the sums to other adders.
It would be desirable to be able to provide programmable logic devices with multiplier circuits, where those multiplier circuits are configured to reduce resource utilization.